Darwin wrote the two-page letter to Ferdinand Vandeveer Hayden to thank him for sending two field surveys of land that would later become Yellowstone National Park, one of the stamps the Postal Service recently dedicated at the World Stamp NY—2016.

In the letter, dated May 2, 1875, Darwin wrote:

“I am much obliged to you for your kindness & for the honor which you have done me in sending your Geological Report of the Yellowstone River & your Preliminary Field Report on the Colorado & New Mexico.”

Darwin is said to have written 7,000 letters in his life, according to Tim Berra, a Darwin expert from Ohio State University.

Another stolen postal treasure story has made the news recently. An Inverted Jenny stamp that was stolen 60 years ago has been returned to the American Philatelic Research Library. Read about the Inverted Jenny stamp here to learn more.